Recently, a large manufacturer of aluminum sheet metal for the automotive industry needed to increase testing capacity in order to meet the needs of quality assurance (QA) testing in support of production. They either had to purchase more frames and the laboratory technicians needed to run them or add automated testing systems. Manually operated testing machines are less expensive, sometimes by a factor of two or three, but the added labor would increase long-term cost and perhaps introduce variability.

It was decided that the best long term solution was to add automated testing capacity, however, the average throughput of a typical metal tensile testing system (≈ 11-12 specimens per hour) was below the minimum needed.

For an automated system, the output capacity (measured in tests per hour) depends largely on the number of interim automated steps (barcode reading, measurement, hardness measurement, tensile testing, and discard). In the case of metals, where tests can last between 1 and 5 minutes, it is generally the length of the tensile test that is the determining factor, which can only really be improved by utilizing closed loop strain control.

In order to meet the production testing demands of 15-20 specimens per hour, it was necessary to evaluate the usage of the specimen handling robot and determine if there was any idle time that could be used for parallel processing additional tasks. During the analysis, it became clear that for these materials it was possible to increase the output per hour by adding a second frame. The system is described as having one central robot, servicing two frames to maximize the use of idle robot time.

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The upper chart, “Single Frame and VSMD”, shows that with a tensile test time of 4 minutes the throughput is about 11 specimens per hour. VSMD stands for Vertical Specimen Measurement Device – which requires the robot to manage the vertical movement of the specimen between the measurement probes, thus using up potential idle robot time.

The lower chart, “Dual Frame and HSMD”, shows the effect of switching to a Horizontal Specimen Measurement Device (HSMD), which is entirely self-contained and does not require the robot to manage the 6 measurements (3 width & 3 thickness). This significantly increase the robot idle time, which made it possible to service the second load frame and increase overall throughput to just under 20 specimens per hour.

In the end, the 2-frame/one-robot system was able to process and test nearly twice the number of specimens as a single frame system – approximately 19 per hour – yet costing $100,000 less than 2 separate systems and less expensive in the long-term since no additional headcount was needed.

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What is It?

Thermo-mechanical fatigue (TMF) is a highly challenging test, critical to the development of advanced turbines. In this test, a material is subjected to repeated cycles of both mechanical strain and elevated temperature. Originally an unusual test used by only a few expert practitioners in highly specialized industry research, this has now come to be an important characterization tool with broad acceptance and established methods.

Why is it Complex?

Derived from high-temperature low cycle fatigue (LCF) testing, this is a better simulation of the operating conditions experienced by materials in turbines and power generation, where in reality strain and temperature vary with a particular synchronization. That synchronization is important since different operating scenarios can have different phases. For example, strain and temperature might increase and decrease in concert, or alternatively one increases first, then the other. This has significant effects on the damage accumulation and performance of any material. Additionally, with changing temperature comes thermal expansion or contraction of the material which must be analyzed and compensated for during the mechanical test, otherwise, the mechanical component of the applied strain may differ greatly from the intended level. Furthermore, heating is most commonly achieved using inductive heating systems, then cooling by air jets, because the required cycle time is only in the region of tens to hundreds of seconds, making conventional furnace technology completely unsuitable. Inductive heating is highly effective, but also highly sensitive to specimen geometry and material. Finally, although LCF is well established, there should be no illusion that it is trivial to do a good test; precise alignment and machine rigidity are essential, as well as highly accurate strain control, which requires careful setup due to the yielding of the specimen during each cycle.

What’s Different with Instron?

Instron Japan originally lead the world in providing integration of inductive heating with mechanical test systems, as Japan took the lead in the early implementation of TMF studies 30 years ago. Since then Instron TMF testing systems now provide a fully integrated, turnkey solution for analysis of combined thermal and mechanical loading cycles on high-performance materials.

Throughout many years of collaboration with industry experts, Instron has recognized our customers’ desire to simplify the process of setting up and running a TMF test. Instron TMF software is designed to be user-friendly and to meet the requirements of ASTM E2368 and ISO 12111. The automated test process reduces the risk of human error caused by manual test configuration. Built-in live graph displays and data post-processing provide effective monitoring and analysis capabilities. This complex area of materials testing still requires high levels of knowledge and expertise, but our user-friendly systems bring previously unseen levels of usability to this sector.

What’s New in the Business?

The last few years have seen a focus on wider industry-specific needs. For example, the aerospace industry is trending towards increased efficiency, meaning turbines with more exotic alloys and thermal coatings and expanding the understanding of TMF behavior of secondary components such as ducts, nozzles, and joints. For the power generation market, more frequent temperature cycles to higher operating temperatures. Most recently, the automotive sector has joined the TMF community as they seek to save processing costs and weight in components around the engine, such as exhaust manifolds.

Now, as our global community – and economy – demands rapid reduction in carbon emissions, there is a clear imperative to accelerate every possible development for better efficiency. This has resulted in a much stronger TMF community, bringing to fruition a new European Code of Practice for force controlled TMF (a key tool for investigation of low strain fatigue, welded joints, and surface features) which is under consideration as the basis for a new ISO standard, and an internationally attended TMF workshop in Berlin, April 2016. It has also lead to new industry and European funding for various programs including development of a code of practice specifically for TMF crack growth (DevTMF program). It is also timely, since both the key international standards for strain controlled TMF (ASTM E2368 and ISO 12111) are starting a periodic review this year, meaning that today’s understanding and capability for such matters as temperature measurement, strain control, and specimen design can be introduced to give better, more prescriptive guidance. Lastly, in response to demand and interest from both the TMF community and industrial LCF testing, the British Standards Institute is leading an ISO project to develop a new standard for verification of temperature measurement during fatigue testing, led by Instron’s Peter Bailey as the convener.

Interested in learning more about TMF testing?

Safety and efficiency are important factors for labs that need to comply with ASTM E23 and ISO 148 impact testing standards. We understand the challenges facing these labs and have incorporated design features of the MPX Series Pendulum Impact Testers that will safely improve the accuracy and throughput in your test lab.

The MPX utilizes unique features to promote a safe testing environment and confidence in results. One of these features is the Automatic Test Start which begins the test when the door to the system closes. This also increases efficiency since the operator can immediately start preparing another specimen after the door is closed.

Repetitive strain injuries are a thing of the past with the MPX due to the motorized lift. The interlocked system also meets strict CE and ISO 13849 requirements, protecting operators from accidental injury as well as the company from the effects of a workplace injury.

Not only is this system safe, it is also accurate. The resolution of the encoder allows the best in class accuracy we have seen versus digital and dial predecessors. This class-leading encoder has the ability to accurately test both high and low energy impacts on one system.